Effective operation of magnetically sensitive instruments, such as electron microscopes and magnetically sensitive circuitry used in metrological applications, require a stable magnetic operational environment. Magnetic field instabilities pose a significant problem to the operational effectiveness of these magnetically sensitive instruments. In some cases, the magnetic instabilities may produce uncertainty in the operating parameters of these instruments or, alternatively, they may tend to prevent reasonable results from being obtained. For example, in the case of an electron microscope, the image being analyzed may be degraded by magnetic instabilities.
Generally, a magnetically sensitive instrument is operated within a given magnetic environment. Such environments may contain DC field components (e.g., the earth's magnetic field); slowly wandering DC field components (e.g., those resulting from nearby elevators or subways); and AC field components (e.g., those caused by power sources or grounding problems). These frequency components may be oriented in a vertical or horizontal direction with respect to the magnetically sensitive instrument and may require neutralization to effectively operate the instrument. Thus, compensation systems have been required for use with magnetically sensitive instruments to control the magnetic fields within the instrument's operational environment.
A typical compensation system generally comprises several components. A magnetic sensor is disposed proximate to the instrument. The sensor senses the magnetic fields in the instrument's operational environment. A filter bank or processor extracts field components having frequencies corresponding to those magnetic fields intended for neutralization. The output of the filter bank or processor is provided to an amplifier for generating a sufficient current to drive a compensation coil to produce neutralizing magnetic fields. The compensation coil produces magnetic fields substantially equal in magnitude and opposite in direction to the sensed magnetic fields to effectively neutralize the magnetic field components intended for neutralization.
Previous compensation systems were generally incapable of compensating for both DC and AC field components. For instance, Buncick (1982), provided a compensation system with a very slow response time. It was only capable of stabilizing DC field components. Whereas, Gemperle and Novak (1976), Gemperle, et al. (1974), and Hadley, et al. (1971), developed compensation systems only capable of detecting AC magnetic fields and, therefore, only compensated for AC frequency components. Compensation for both AC and DC magnetic field instabilities was provided by Hand (1976), but such result was accomplished only by simultaneously using two compensation systems.
More recently, compensation systems have taken advantage of the capability of fluxgate magnetometers for detecting both DC and AC magnetic field components in a particular environment. These compensation systems are, therefore, capable of compensating for both AC and DC frequency components in the same system. However, such compensation systems require dedicated hardware in the form of tuned filter-amplifier pairs in which each filter-amplifier pair must be tuned to a particular frequency component present in the magnetic environment. (See Luzzi, U.S. Pat. No 5,225, 999, issued on Jul. 6 1993) Such tuning requires the relevant magnetic environment to be evaluated to determine which frequency components are significant in terms of the instrument's operational performance. After identifying significant field components, the number of filters-amplifier pairs required by the compensation system is determined and the band-pass frequencies of those filters can be preselected.
The disadvantages of such systems are apparent when the number of significant field components to be neutralized is large. For example, the expense and complexity of the system increases with the number of components. The phase delay associated with each separate filter-amplifier pair requires a separate timing adjustment to ensure that the neutralizing magnetic field is produced with an accurate phase to effectively neutralize the sensed magnetic field.
Additionally, each environment requires careful advance analysis to determine significant field components. However, the significant field components may vary over time and may differ depending upon each instrument's particular sensitivity. Thus, once a system is designed for a particular instrument to be operated in a specific environment, it may not be operable for other instruments or when changes occur in the operational environment. Moreover, such systems are incapable of compensating for broadband disturbances caused, for example, by switching motors on or off, since the occurrence and characteristics of such broadband disturbances are random.
Other compensation systems have addressed some of these problems by implementing the signal analysis components in software. For example, Integrated Dynamics Engineering, Inc. (ADA) located in Woburn, Mass., has developed a compensation system using a digital signal processor (DSP) instead of a plurality of filter-amplifier pairs. The DSP is pre-programmed with data identifying the significant field components present in the instrument's operational environment that are to be neutralized. These compensation systems can be adaptively modified for use with different instruments and may be easily modified in response to changes in the instrument's operational environment. However, the ADA system still requires a costly signal processor and an a priori analysis of the operating environment to appropriately pre-program the DSP. Thus, the ADA system compensation is limited to only two frequencies and four harmonics.
Many available compensation systems utilize a Helmholtz coil or a semi-Helmholtz coil (e.g., each of the two loops of the Helmholtz coil may be non-circular) to serve as the compensation coil. Typically, each coil loop is placed around the perimeter of a room in which the instrument is to be used. To compensate for vertical magnetic fields, one of the coil loops is located near the floor of the room and the other coil loop is located near the ceiling so that the instrument can be disposed within the Helmholtz coil. Similarly, to compensate for horizontal magnetic fields, each coil loop of the Helmholtz coil is located near opposite walls of the room.
The coil loops used in compensation systems generally comprise a number of turns of wire. However, each additional turn adds inductance to the system thereby increasing the circuit delay, the system expense and complexity.
Therefore, there exists a need for a simple and cost-effective magnetic compensation system to be used during the operation of a magnetically sensitive instrument which is sufficiently adaptable for neutralizing both DC and broadband AC magnetic field components independent of changes in the operating environment of the magnetically sensitive instrument and independent of the type of magnetically sensitive instrument to be operated.